Recently, a constant-Internet-connection environment has been rapidly widespread, both at companies and general households, as a result of an improved broadband environment provided by ADSL, optical fiber, and so on. In addition, not only personal computers (PCs) but also household appliances, such as televisions, DVD recorders, air conditioners, and refrigerators, are continuously connected to the Internet and keep obtaining contents from a server through which they are connected to the Internet, while such household appliances communicate with each other, so as to give and receive, and share information.
For communications between apparatuses connected to the Internet, global IP addresses, which are assigned uniquely to the respective apparatuses, are used. However, due to a rapid increase in the number of apparatuses connected to the Internet, the number of global IP addresses tends to be insufficient. Therefore, PCs and household appliances that are not assigned with global IP addresses are connected to a router assigned with a global IP address, so that a Local Area Network (LAN), which is not directly connected to the Internet, is established. In the LAN, private IP addresses, which are unique only within the LAIN, are often used, and such private IP addresses are used only for communication taking place within the LAN.
On the other hand, private IP addresses are not unique on the Internet, and the use of such private IP addresses is not permitted on the Internet. Therefore, an apparatus having a private IP address cannot, by itself, communicate beyond the router with another apparatus connected to the Internet.
Problems like this can be solved with the Network Address Translation (hereinafter, referred to as NAT) function and the Network Address Port Translation (hereinafter, referred to as NAPT, and otherwise referred to as IP masquerade) function, which allow an apparatus assigned with a private IP address on the LAN to communicate with another apparatus on the Internet, through reciprocal conversion between private IP and global IP addresses. Note that the NAT function and the NAPT function have the same operating characteristics, and therefore are generically referred to as the NAT function where it is not necessary to distinguish these functions.
A router equipped with the NAT function (hereinafter, described as a NAT router) is not only used for connecting an apparatus having only a private IP address to the Internet but also widely used for connecting an apparatus making up a specific LAN and having only a private IP address with an apparatus making up another LAN.
Such a state is a connection mode commonly seen in cable television, in apartment Internet, and so on. A subscriber to cable television or the like has only a private IP address assigned by the cable-television side. That is, the cable-television side treats the subscriber's apparatus as part of the LAN connected to the NAT router owned by the cable-television side. Thus, in the case where the subscriber wants to connect plural apparatuses, the subscriber buys another NAT router and connects the apparatuses to the NAT router, so that the service is provided. As a result, a network structure using the Internet as a route and hierarchized with plural NAT routers is formed.
Note that in the Description and Claims, in order to avoid misunderstanding of terms, what is known as the Internet through which apparatuses having global IP addresses communicate with each other is referred to as the Internet, and from the perspective of the NAT router, the network on the side able to communicate through the Internet is referred to as a WAN-side network; from the perspective of the NAT router, the downstream network, which cannot be directly connected to the Internet, is referred to as a LAN-side network; and, from the perspective of a given apparatus, the network to which apparatuses are connected within the range not exceeding the NAT router is referred to as a local network. In addition, from the perspective of the NAT router, the address on the Internet side is referred to as a WAN-side address, and from the perspective of the NAT router, the address on the local-net side is referred to as a LAN-side address. The WAN-side address is equivalent to a global IP address in the case where the NAT router is directly connected to the Internet, and is equivalent to a LAN-side address set by the NAT router connected directly upstream in the case where the NAT router is not directly connected to the Internet.
FIG. 3 shows an example of a hierarchized network structure assumed in the present invention. Note that FIG. 3 does not necessarily show a conventional example, and that, in particular, the server 205 in FIG. 3 is a server according to the present invention.
A shared-facilities room 214, a shared wiring area 215, a house 1 (216), and a house 2 (217) are provided with apartment-network facilities. In the shared-facilities room 214, a NAT router 209 is provided, which is connected to the Internet 210 through an access line, and a global address is assigned to the WAN-side of the NAT router 209. The NAT router 209 distributes an arbitrary number of private IP addresses within an apartment building, according to a DHCP method or the like.
In the house 1 (216), a NAT router 206, which is directly connected to the NAT router 209, is provided. The LAN-side of the NAT router 206 is connected, through an intra-home LAN 211, to an apparatus 201, an apparatus 202, and a NAT router 207; to the NAT router 207, an apparatus 203 is connected. Inside the house 1 (216), a network using plural routers is established.
Meanwhile, in the house 2 (217), a NAT router 208 is provided, with its WAN-side being connected to the NAT router 209, and its LAN-side being connected, through an intra-home LAN 213, to an apparatus 204.
In such a network, which is connected and structured using NAT routers, there are two major problems regarding the operation of the NAT routers when a connection between arbitrary apparatuses is attempted. The first problem is that it is not possible to transmit a packet, when requested by the WAN-side, toward an apparatus belonging to the LAN-side of a NAT router. The other problem is that it is not easy to identify the WAN-side address of the NAT router to which the apparatus belongs. Patent Reference 1 discloses the details and problems of the NAT function that are described above.
Conventionally, the methods for connecting a given apparatus to an arbitrary apparatus within a network structure in which a NAT router is connected to the Internet include a method which combines the functions of an address registration server and a Universal Plug-and-Play Internet Gateway Device (hereinafter, referred to as UPnP-IGD). In the method, it is assumed that the NAT router is equipped with the UPnP-IGD function. The UPnP-IGD is a de-facto standard issued by the UPnP Forum, and equipped on a number of NAT routers. In the method, an apparatus in the LAN-side network can access the NAT router to which the apparatus is directly or indirectly connected and call the UPnP-IGD function, and refer to or setup a static NAT table in the NAT router. With this, it becomes possible to automatically set the static NAT table in the NAT router, without being through complicated manual settings by a user, so that the apparatus in the LAN-side network can automatically start communication, toward the apparatus itself, from the WAN-side. Furthermore, the method allows identification of the address of a connection-partner apparatus before starting the connection, by registering, in the address registration server, the WAN-side IP address of the NAT router that is obtained by UPnP-IGD function, or the Internet-side address of the NAT router that is detected by the server. An exemplary case where such a method is implemented is the Microsoft Messenger Service, which permits communication, file transfer, and other functions.
In addition, Patent Reference 2 discloses a technique for judging, through matching of global IPs, whether or not the apparatuses are connected to an identical NAT router, when communicating on the Internet or in the LAN through the above-described NAT router.    Patent Reference 1: Japanese Patent No. 3445986.    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2004-173240.